The proposed research will examine the mechanism of the reaction catalyzed by the enzyme Lecithin: Cholesterol Acyltransferase (LCAT). Interaction between the enzyme and high density lipoprotein (HDL), its primary substrate in vivo, will be investigated. A newly developed method involving hydrophobic chromatography will be used for the isolation of HDL. This technique avoids some of the compositional changes that are associated with other preparative techniques, particularly ultracentrifugation. The HDL will be fractionated on DEAE cellulose into subfractions which are known to exhibit differential reactivity toward LCAT. Specific phases of the proposed research will include: 1) Incubation of the HDL subfractions with purified enzyme and examination of the LCAT reaction as a function of the concentration of substrate lipids and those of specific apoprotein components of the lipid/protein substrate complex. 2) Study of the transfer of substrate lipids from triglyceride rich lipoprotein to HDL subfractions and the effect of the attendant changes on the LCAT reaction. 3) Study of the mechanism of product (lysolecithin) removal from the enzyme surface. The LCAT/HDL system has been implicated in the process called reverse cholesterol transport which facilitates the transfer of excess cholesterol from peripheral tissues to the liver. It is thus likely that the findings of the research proposed here would aid in the understanding of the reversal of atherosclerosis. Consequently, the findings may ultimately be useful in developing modalities for the prevention of cardiovascular diseases including coronary heart disease.